This invention relates to a system for reading coded information which is expressed by a pattern such as a bar code pattern and, more particularly, to an improvement of a reading system of a type in which a pattern is formed with magnetically different materials and this pattern is read by an electromagnetic sensor.
Known in the art is a coded pattern reading method according to which a black and white bar code pattern in which light reflection property varies depending upon the black and white portions is read in an optical manner. The optical type coded pattern reading method has the disadvantage that, when reflection and permeation of light are hindered due to deposition of dust on the black and white pattern section or the reading unit, it becomes extremely difficult to detect the pattern.
The prior art optical type bar code system can be applied only to a case where an article on which a pattern should be provided is made of a material such as paper on which the pattern can be printed or an article on which a printed pattern paper can be bonded semifixedly so that it will not come off easily. It is difficult to apply this optical type bar code system to other articles. If, for example, one attempts to apply the optical type bar code system to a case where a manufacturing number should be affixed in the form of a pattern to an article made of metal such as cast iron, he will not be able to print an optical type bar code pattern on the cast iron itself. Bonding of a paper having a pattern printed on it on the cast iron article will not work because it will easily come off from such cast iron article and/or become soiled.
U.S. Pat. No. 4,883,949 proposes a magnetic type coded data reading device according to which a patterned code representation magnetic sheet is bonded to an article and coded information on the code representation magnetic sheet is read by means of a magnetic sensor employing a mutual induction coil comprising a primary coil which is excited by ac and a secondary coil which thereby generates a mutual induction voltage. According to the code representation magnetic sheet to be read by this coded data reading device, coded information can be read magnetically even when it cannot be read optically because the pattern surface is soiled with dust. In this coded data reading device, the code representation magnetic sheet which is prepared separately must be attached to the article semi-fixedly to prevent ready coming off of the sheet from the article as in the case of the optical type reading device; however, there still remains the likelihood of coming off of the magnetic sheet.
This prior art coded data reading device identifies coded information by operating an amplitude value of mutual induction voltage produced in the secondary coil of the magnetic sensor by a microcomputer or the like computing device, utilizing the phenomenon that the amplitude level of mutual induction voltage produced in the secondary coil of the mutual induction coil varies with the width, thickness and interval of a metal portion of the code representation magnetic sheet. Since, however, the amplitude value of mutual induction voltage produced in the secondary coil varies also with the distance (gap) between the code representation magnetic sheet and the mutual induction coil, a protection film is normally provided on the surface of the magnetic sheet which constitutes a guide surface for the mutual induction coil to provide a flat surface with a view to maintaining the distance constant during reading of the coded information. Accordingly, there arises the problem that, when the surface of the magnetic sheet has become uneven due to unintended deposition of paint or the like on the surface of the magnetic sheet after the magnetic sheet has been bonded to the metal material such as case iron, the distance between the magnetic sheet and the mutual induction coil is changed, with the result that accurate reading of the coded information becomes difficult. Further, since the amplitude level of the mutual induction voltage changes under the influence of an environmental change such as change in the ambient temperature, this factor also adversely affects the accuracy in reading of the coded information.